As a representative of sound emitted from a vehicle, there is exhaust sound emitted from an exhaust system of a vehicle. As for the exhaust sound, such results as being indicated by sound quantity (sound pressure) and its frequency distribution as its physical quantity are evaluated, and an effective counter measure has been provided for a muffler or the like disposed in the exhaust system, in order to reduce it to be equal to or smaller than a reference value of the sound quantity determined according to laws or the like. On the other hand, tone quality (timbre) of the exhaust sound may affect the impression of the vehicle, so that it can be said that the tone quality of the exhaust sound is one of factors affecting marketability of the vehicle.
With respect to the tone quality of the exhaust sound, such a subjective ranging method had been employed in the past, as expressing impression to a subject in audition thereof with words like “brightness” and “sharpness”, or scoring it. For example, in Non-Patent document 1, there are disclosed relationships between first and second formants and vowel sound, and it is indicated that the formants are strongly correlated with the vocal vowel sound. However, as it was difficult to indicate the tone quality as the physical quantity, the subjective ranging method was lack of objectiveness and quantitative, and furthermore it was difficult to evaluate appropriately the tone quality of the exhaust sound varied according to driving conditions.
Even in that situation, a try for objectively evaluating the tone quality (timbre) of the exhaust sound was made, and various methods have been proposed from the past. For example, in Patent documents 1 and 2, there are disclosed methods for estimating and evaluating the tone quality from the sound level at every frequencies, and in Patent document 3, there is disclosed a methods for estimating and evaluating the timbre from fluctuation of the sound level at every frequencies.    Patent Document 1:    Japanese Patent Laid-open Publication No. 6-186077    Patent Document 2:    Japanese Patent Laid-open Publication No. 7-306087    Patent Document 3:    Japanese Patent Laid-open Publication No. 6-117912    Non-Patent document 1: Human Voices and the Wah Pedal, Copyright 1999 R. G. Keen [online]. [retrieved on 2005-06-14]. Retrieved from the Internet:    <URL:http://www.geofex.com/Article_Folders/wahpedl/voicewah. htm>